1. Field of the invention:
This invention relates to a field effect transistor and the method of making same, and more particularly to a junction type field effect transistor having unsaturated characteristics and the method of making same.
2. Description of the prior art:
The depletion layer has long been investigated and utilized in semiconductor devices. But, there still remain properties not fully analyzed. Thus the concept of the depletion layer will first be described briefly.
When a p-type region and an n-type region are located contiguous to each other in a semiconductor bulk, a pn-junction is formed, whereat the respective carriers (holes and electrons) are quite different in their densities from one side to the other side, the hole density being much larger in the p-type region than in the n-type region and the electron density being much larger in the n-type region than in the p-type region. Therefore, in the vicinity of the pn-junction, the holes tend to move from the p-type region into the n-type region across the pn-junction while the electrons travel from the n-type region into the p-type region. Thus, a negatively-charged portion is formed in the p-type region near the pn-junction. Likewise, a positively changed portion is formed in the n-type region near the pn-junction. Thus, there is formed an electrical double layer. Once such a double-layer is formed, it establishes a potential gradient to prevent further inter-regional travels of charge carriers. Such a double-layer region is called a space charge region or a depletion layer. Such depletion layers have been effectively utilized in field effect transistors.
In conventional junction type field effect transistors, carrier channels, where charge carriers are transported, are usually defined and controlled by a depletion layer extending from a pn-junction surrounding the gate region. Such conventional field effect transistors, however, showed saturated characteristics in the drain current vs. drain voltage relation.
Recently, an unsaturated, triode-like field effect transistor has been proposed (e.g. U.S. Pat. No. 3,828,230 to Nishizawa & Terasaki). In this unsaturated type field effect transistor, the depletion layer extending from the gate region is designed to close the so-called current channel even at null gate bias. Thus, the charge carriers should penetrate through the depletion layer in going from the source to the drain in every operable state. The charge transfer in the depletion layer was considered just as that in vacuum. The current capacity was considered to be proportional to the cross section of the current channel. According to this concept, for achieving a large current capacity field effect transistor, the channel area should preferably be as large as possible. For closing such a large channel area, the carrier concentration in the current channel should be less than a certain value and preferably be as low as possible. In other words, the lower the impurity concentration in the semiconductor region which forms a current channel is, the larger the current that would be allowed to pass.
When one applies the above concept to the vertical multi-channel junction type field effect transistor as proposed in U.S. Pat. No. 3,381,188, it would be possible to provide a large power active elements. But an increase in the cross-section of each current channel should be accompanied by a corresponding decrease in impurity concentration and leads to a larger internal resistance, i.e. drain resistance, and a smaller amplification factor.
Further, the use of an extremely pure semiconductor material makes the practical manufacturing process of devices very difficult because a very little amount of the extrinsic material may wholly change the electric characteristics of the material.
Further the zero gate bias characteristic curve in the drain current to drain voltage relation according to the above structure is separated from the current axis by a considerable amount. This means a high "ON" resistance and a loss in power.
This invention is to solve these problems and to provide a practically useful structure for an unsaturated type field effect transistor which is easy to manufacture, and a method of making same.